A major focus of this year's work on the U.S. 395 North Spokane Corridor project has been construction of an innovative structure that will carry traffic over the Burlington Northern Santa Fe railroad track just east of the Market Street-Hawthorne Road intersection.
The structure is a quarter-mile-long concrete arch tunnel that Scarsella Brothers Inc., Seattle-based contractor for the job, built over the curving section of BNSF tracks. The tunnel will be backfilled so the highway, which crosses the tracks at an angle, can be built over the top of the tunnel.
According to a publication by Contech Construction Products, WSDOT initially considered building a conventional girder bridge over the tracks but began looking at alternatives when the structure's size and cost became an issue. Prefabricated tunnel options included either concrete or a steel plate arch system. In the end, Contech said, BNSF accepted a BEBO precast concrete arch system.
The BEBO structure not only offered the best crash worthiness in the event of a train derailment inside the tunnel, it also allowed the tracks to remain in service during construction, said a spokesperson for HDR Inc., which provided engineering for the project.
The BEBO Concrete Arch System is a combination of cast-in-place concrete footings, precast arch elements, headwalls, and wingwalls. Developed in 1966 in Switzerland, the BEBO Concrete Arch System is a precast concrete version of the arches used in Roman aqueducts built two millennia ago – which are still standing. The system uses the fundamentals of soil-structure interaction to achieve strength and stability. The results are the largest precast arch spans in the world.
Built on a Curve
The Washington State Department of Transportation executed the $36.8-million contract with Scarsella Brothers to build the tunnel on Dec. 17, 2007. Though winter weather delayed the beginning of dirt work on the job site until spring 2008, fabrication of the precast tunnel sections at Central Pre-Mix Prestress Plant in Spokane began just nine days after the contract was executed.
The structure consists of approximately 444 precast concrete segments, each of which is 6 feet long. The tunnel will be 1,330 feet in length and 54 feet in width – wide enough to accommodate a second railroad track in the future – and has a 25-1/2-foot rise.
Central Pre-Mix was able to pour an average of five quarter-round arch pieces per day. Because the tunnel is curved, each of the BEBO units is slightly pie-shaped to accommodate the alignment. They weigh about 44,000 pounds apiece.
“The arch pieces are different sizes to allow for the radius,” said Mike Fletcher, superintendent for Scarsella Brothers on the project. “They are all numbered, and every panel has a certain place where it goes.”
Construction on the site started March 10, 2008. Scarsella Brothers is self-performing the dirt work, which includes clearing and grubbing, two temporary BNSF crossing installations, constructing a haul road, and footing excavation work, as well as backfilling the tunnel.
Eight Caterpillar 613 scrapers perform most of the earthmoving for the project. Fletcher said there are seven borrow sites for the project with a maximum haul of about half a mile.
“Most of the hauls are within a quarter of a mile,” he added.
Concrete placement began with the first of 14 pours to create the footings on either side of the tracks that support the arch pieces of the tunnel. These walls measure 19 inches wide by 4 feet deep, Fletcher said.
Clearwater Concrete Inc., McCall, ID, is the subcontractor for tunnel construction, with Crane Construction NW Inc., Sandpoint, ID, doing the lifting. Central Pre-Mix is also the concrete supplier for the walls.
The Clearwater crew used three sets of forms. The east wall was built first, Fletcher said, and then the tracks were shifted over a weekend so Scarsella's crew could dig out the other side. The trains observe a speed limit of 25 mph through the work area, passing once during daytime and once at night each day.
“The railroad has been really good to work with,” Fletcher observed.
Placement of the arch pieces began in August while the last wall sections were still under construction. Two of Lampson International's familiar blue Manitowoc 4000W Vicon crawler cranes were employed to pick the massive BEBO units. Stationed one on either side of the tracks, outside the footing walls, the cranes carefully fitted the bottom end of each piece into the slot in the top of the wall and brought the other ends together at the top of the arch. While workers on the footings pounded wooden wedges into the slots to precisely shim the arch units, others in manlifts at the top of the tunnel bolted the two pieces together. The final process was to grout the joints, top and bottom. Then the process started over.
Fletcher said it was very important to maintain a consistent half-inch gap between all the arches. Unless the work is done with precision, he explained, the last 6-foot-wide arch would be either too long or too short to fit the footings correctly.
Once the crews got up to speed, they were able to place up to 14 arches per day, Ferguson said. Based on that, he figured it would take about 20 working days to complete the concrete tunnel.
By Sept 18, the arch placement was complete, and work was continuing on backfilling and concrete walls. Fletcher said that eventually 25 feet of earth will cover the tunnel, which helps account for the 670,000 cubic yards of dirt work involved in the project.
According to Contech, Spokane's new railroad tunnel will be the longest BEBO structure installed in the United States when it's completed in fall 2009. In addition to building the tunnel, the project also includes mainline paving between Piper Road and the NSC/Market Street Overcrossing.